`--exec-type=docker` Launches Only a Single Process, Ignores `--num-accelerators`

When running mlpstorage training run with --exec-type=docker and --num-accelerators=6, only one process is executed, even though multiple accelerators are specified:

[OUTPUT] Running DLIO [Training & Checkpointing] with 1 process(es)

The benchmark command generated:

/local/.venv/bin/dlio_benchmark workload=unet3d_h100 \
++hydra.run.dir=/mnt/array/j25/result_j16/run7/training/unet3d/run/20250812_115117 \
++hydra.output_subdir=dlio_config \
++workload.dataset.num_files_train=21000 \
++workload.dataset.num_subfolders_train=70 \
++workload.reader.read_threads=26 \
++workload.reader.odirect=True \
++workload.reader.prefetch_size=1 \
++workload.workflow.profiling=iostat \
++workload.workflow.hydra_logging=enabled \
++workload.workflow.job_logging=enabled \
++workload.dataset.data_folder=/mnt/array/j25/data/unet3d \
--config-dir=/local/.venv/lib/python3.10/site-packages/configs/dlio

Works as Expected with --exec-type=mpi

Using MPI does correctly spawn all 6 processes:

mpirun -n 6 -host <ip1>:6 /local/.venv/bin/dlio_benchmark workload=unet3d_h100 \
++hydra.run.dir=/mnt/array/j25/result_j16/run7/training/unet3d/run/20250812_121415 \
...

Output:

[OUTPUT] Running DLIO [Training & Checkpointing] with 6 process(es)

Additional Observation:

In version 2.0 of the code, the logic for wrapping the benchmark command with mpirun is only applied if exec_type == mpi, but bypassed entirely when exec_type == docker.

Relevant code snippet:

if self.args.exec_type == EXEC_TYPE.MPI:
    self.logger.debug(f'Generating MPI Command with binary "{self.args.mpi_bin}"')
    mpi_prefix = generate_mpi_prefix_cmd(
        self.args.mpi_bin, self.args.hosts, self.args.num_processes,
        self.args.oversubscribe, self.args.allow_run_as_root,
        self.args.mpi_params, self.logger
    )
    cmd = f"{mpi_prefix} {cmd}"

It seems the assumption might have been that Docker manages process launching internally but it doesn't. This causes it to default to a single process.

Start in the version 2.0 command-construction logic around the exec_type check and generate_mpi_prefix_cmd. Verify how Docker runs the generated benchmark command with num_accelerators set, then confirm that the Docker path launches all requested processes rather than one.